Most mixed martial arts (MMA) fighters begin their careers at a young age. Yoel Romero, meanwhile, didn’t have his pro debut until he was 31 years old. He is now 40 and looks better physically than most fighters 10 years younger than him and can still compete at the highest level.

Still, underneath it all, “Soldier of God” is hurting, as three decades of non-stop training and competing in sports has taken a toll on his body. During a recent interview on “The Joe Rogan...